[Syntax Breaking] Rename DefaultImpl to AutoImpl

DefaultImpl is a highly confusing name for what we now call auto impls,
as in `impl Send for ..`. The name auto impl is not formally decided
but for sanity anything is better than `DefaultImpl` which refers
neither to `default impl` nor to `impl Default`.

src/librustc_driver/test.rs

@@ -253,7 +253,7 @@ impl<'a, 'gcx, 'tcx> Env<'a, 'gcx, 'tcx> {
                 hir::ItemUnion(..) |
                 hir::ItemTrait(..) |
                 hir::ItemImpl(..) |
-                hir::ItemDefaultImpl(..) => None,
+                hir::ItemAutoImpl(..) => None,
 
                 hir::ItemMod(ref m) => search_mod(this, m, idx, names),
             };